"Fuel of standard formula shall be supplied by the contest organizers. .....................
The fuel shall contain 10% nitromethane, 20% lubricant, and 70% methanol by volume.
If the contestant uses this fuel, the length of the passage through which the exhaust
exits the engine is not restricted."
This applies to the "rulebook" events - if someone is flying a 'local' event then it would
be up to the organizers.
BUT ... the muffler pressure (as pictured) is not legal in 'rulebook' events.